Shape-memory alloys (SMAs) are materials that, once deformed, return to their original shape upon heating. SMAs are typically either of the copper-aluminum-nickel system or nickel-titanium system of alloys, although other alloys may also exhibit SMA properties. Although not the cheapest SMAs, nickel-titanium alloys, such as Nitinol, are popular due to their stability and superior cycling properties.
SMAs function by shifting back and forth between can two different phases, with three different crystal structures (i.e. twinned martensite, detwinned martensite and austenite) and six possible transformations. Nitinol, for example, changes from austenite to martensite when cooled, with a specific transition temperature to martensite upon cooling and specific temperatures upon which the transition to austenite begins and finishes upon heating. Repeated cycling of SMAs eventually leads material fatigue, as evidenced by a creep or drift in the specific transition temperatures. Further, if heated beyond a maximum threshold temperature, SMAS lose their ability to be cycled between shapes and thus are susceptible to permanent deformation. The phase transition from the martensite to austenite is thus a function of temperature and induced stress, but not a function of time.
SMAs may remember only a ‘cold’ shape, to which a deformed SMA returns upon heating and then cooling, or they may remember both a ‘hot’ shape and a ‘cold’ shape, between which they may be thermally cycled. In either case, accumulated cycling will eventually result in fatigue. SMAS do not have infinite cycling capacity and thus potentially long service lifetimes limited by the number of cycles experienced and the degree of stress experienced during each cycling event.
Currently, stress in SMAS is managed with electronic sensors or switches. Nitinol, for example, transforms its crystal structure between martensitic and austenite, with the transformation based on the energy state of the material. Temperature is a common way to measure the energy state of the material. Many devices limit the energy of their SMA components by monitoring the temperature of the SMA element and regulating energy input based on the measured temperature. Alternately, some systems regulate energy input based on percent deformation of the SMA element, such as by using complex algorithms based on experimental data. Both of these methods require some sort of intelligence to manage. Excess energy in the SMA element will degrade the reversible transformation life cycle. As both of the above-described methods measure temperature and/or deformation as a basis for regulation, they both suffer from the drawback that excessive temperature and/or deformation must be measured before regulation is initiated, but if temperature increases or excessive deformations occur rapidly, the SMA element incurs life-cycle shortening damage before regulation is effectively implemented. Thus, there remains a need for a system of regulating SMA element cycling that prevents excessive heat and/or deformation from occurring. The present novel technology addresses this need.